The ISP I work for has had problems getting blacklisted due spam coming from the network, junk email sucks as well. So......  This is an attempt at limiting the exposure to this problem without blocking mail ports all together. Expanding on Butch Evans posted Mikrotik Filters. This idea was shamelessly borrowed from his work as well as finding the exact text word for word posted on a dozen more blog sites each claiming that it was their own.

StarOS simple interface NAT will lead to some problems with Google, Yahoo, MSN and others. These services recognize the 2000 users we have in our network as one IP and blocked them, limited their connection or presented CAPTCHA. To resolve this problem we added 32 IP addresses to the Internet side of the StarOS NAT router and added individual statements to the fire wall dividing our internal network into smaller slices only exposing 1024 host per external IP.